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BENZILIC ACID                                203

Benzil, C0H5CO.CO.C6H6
15 grms. benzoin.                                                                            *
35     „     cone, nitric acid, sp. gr. 1-4.
The benzoin and nitric acid are heated on the water-bath with                       '    ,
an air condenser, the flask being occasionally shaken. Nitrous
fumes arc evolved, and the crystals of benzoin are converted
into a yellow oil, which, after two hours' heating, is free from un-
changed benzoin. The contents of the flask are now poured
into water, and the yellow crystalline deposit separated by
filtration, washed with water, and recrystallised from alcohol.
Yield, 10—12 grams.
Properties.— Yellow prisms;  m. p. 95°; insoluble in water;                         '«
soluble in hot alcohol.                                                                                           * \>
Reaction.—i. Dissolve a smalt quantity of benzil in a little
alcohol, acid a fragment of caustic potash and boil. A violet
solution is obtained.                                                                                             , '
Benzilic Acid, (C6H5)2C(OH).CO2H                                      '   ,
10 grms. benzil.                                                              4
50     „     caustic potash.
The caustic potash is melted with a small quantity of water in a                      ,
silver or nickel crucible. The temperature of the mass is brought                     ;M
to 150°, and the finely powdered benzil added.   The benzil melts,                     '•'
and the mixture shortly changes to a solid mass of potassium                       ffj
benzilate.     The  cooled  melt  is   dissolved  in water, and the                       jg
alkaline solution acidified with hydrochloric acid, which precipi-                     fiti
tates the bcnzilic acid.    The crystalline mass, which contains          .              '^'
small quantities of benzoic acid, is separated from the mother-                         t(l^
liquor and washed with cold water.    It is then transferred to a                         *.,^
porcelain basin, dissolved in hot water, and the solution boiled                         n >,'
until the smell of benzoic acid has gone.    On cooling, benzilic                         ^
acid crystallises out, and is purified by a second crystallisation                          l|
from hot water.                                                                                                               )<.>'
C«Hfi.CO.CO.CflHfi+ KOH =(C0H5)aC.(OH).COOK.                                    !|
/^-^;'//^.—Colourless needles ; m. p. 150° ; scarcely soluble                         |§
in cold, readily in hot water and alcohol.                                                                  ||*
Reaction.—Add a little concentrated sulphuric acid to benzilic                         i||
acid.    It dissolves with an intense red colour.    See Appendix^                         M
P- 3°3-                                                                                                       U